The introduction and widespread acceptance of intelligent workstations has changed the data processing paradigm from centralized host computing to distributed processing. Also, with the growth of distributed processing, the need for LAN interconnection and the growing use of graphics and images has lead to exponentially increasing network traffic. Furthermore, not only has the demand for connectivity changed, the technology to provide networking facilities has changed as well. Although newly introduced digital and fiber technologies provide faster and more reliable communication, they require networking techniques which are able to operate efficiently at higher speeds. In order to meet this requirement, fast packet switching has been developed.
Frame relay is one kind of fast packet switching. Because fast packet switching techniques such as Frame Relay operate below layer 3 of the OSI model, they readily accomodate multiple higher-layer protocols, and in particular the IP protocol.
A Frame Relay network provides a number of Permanent Virtual Circuits (PVC), which form the basis for the connections between stations attached to the network. The PVCs allow data exchange between these stations. The resulting set of interconnected devices is called the Frame Relay group, which may be either fully interconnected to form a fully meshed network, or only partially interconnected to form a partially meshed network. In either case, each PVC is uniquely identified at each Frame Relay interface by a Data Link Connection Identifier (DLCI). The DLCI is different on either end of the PVC, and consequently has strictly local significance at each interface.
A fully meshed Frame Relay network provides optimum connectivity. In the IP configuration, the whole network is seen as a single IP subnet. This configuration has no connectivity limitations since any router can reach all other routers. Unfortunately, a high number of PVCs are required, and the number of PVCs increases significantly whenever a new router is added to the network.
Generally, the mapping between the IP addresses of the routers and the DLCIs employs an inverse Address Resolution Protocol (ARP) table associated with the router. To update its inverse ARP table dynamically a router sends or receives requests over a PVC. In such requests, the known hardware address is the DLCI corresponding to the router end. When receiving either an ARP request or a reply to an ARP request over the PVC, the router can associate, in its inverse ARP table, the IP address (as entry) of the device at the other end of the PVC with the DLCI being used. Since a fully meshed network is seen as a single IP subnet, and since any router has PVC connectivity to all other routers in such a network, a router can dynamically map the remote IP address-to-DLCI using the inverse ARP method.
Partially meshed networks can be made of several IP subnets wherein one router, the hub, has a PVC connected to each of the other routers of the subnet, the spokes. In such a case, spoke-to-spoke connectivity is resolved via IP subnet-to-subnet connectivity, which is the normal IP routing process. When a spoke wants to reach another spoke of another subnet, it will use its routing table, which indicates a route via the hub.
This method requires a different IP subnet per PVC, which may result in IP-address exhaustion. It also creates very large routing tables because of the number of new subnets. These large routing tables may cause memory problems inside the routers, as well as high bandwidth utilizations between the links when exchanging the routes for these subnets. Partially meshed networks can also be a single subnet. In that case, however, a dynamic inverse table cannot be used, as it does not provide spoke-to-spoke connectivity.
One solution to the above problem is to do the mapping manually. This means that the inverse ARP table is manually configured with the IP addresses of all the spokes and the corresponding DLCIs. Unfortunately, such a solution, which must be implemented for all the spokes, becomes burdensome when many spokes are present in the network.